Hadith 2:93

Narrated Al-Bara':

The Prophet went towards Al-Baqi (the grave-yard at Medina) on the day
of Id-ul-Adha and offered a two-Rakat prayer (of 'Id-ul-Adha) and then
faced us and said, "On this day of ours, our first act of worship is
the offering of prayer and then we will return and slaughter the
sacrifice, and whoever does this concords with our Sunna; and whoever
slaughtered his sacrifice before that (i.e. before the prayer) then
that was a thing which he prepared earlier for his family and it would
not be considered as a Nusuk (sacrifice.)" A man stood up and said,
"O, Allah's Apostle! I slaughtered (the animal before the prayer) but
I have a young she-goat which is better than an older sheep." The
Prophet (p.b.u.h) said to him, "Slaughter it. But a similar sacrifice
will not be sufficient for anybody else after you."